Apparatus and method of measuring interference

ABSTRACT

A receiver comprising means for receiving a first plurality of signals at different frequencies at substantially the same time; and means for comparing a wanted signal at a first frequency with at least one other signal at a frequency similar to that of said wanted signal to provide a measure of interference provided by said at least one other signal.

FIELD OF INVENTION

[0001] The present invention relates to a method and apparatus formeasuring interference, in particular but not exclusively, for usewithin a cellular telecommunications network.

BACKGROUND TO INVENTION

[0002] In known wireless cellular telecommunications networks, an areacovered by the network is divided into a plurality of cells. Each ofthese cells has a base station which is arranged to transmit signals toand receive signals from mobile stations located in the cell associatedwith the respective base station. Mobile stations will be in activecommunication with the base station associated with the cell in whichthe mobile station is located.

[0003] Each base transceiver station is, in an example of the GSM(Global System for Mobile Communications) standard, arranged to receiveN frequency channels out of a possible 125 frequency channels F₁. . .F₁₂₅ available. The total bandwidth occupied by the 125 frequencychannels is 25 MHz which means that each frequency channel has abandwidth of 200 kHz. Other bandwidths between 4.8 and 30 MHz exist.Each channel is divided into frames each of which are furthersub-divided into time slots. The GSM standard is a time divisionmultiple access (TDMA) system and accordingly different mobile stationswill be allocated different time slots. The base transceiver stationwill therefore receive signals from different mobile stations indifferent time slots using the same frequency channel. N is usually lessthan 125. Each base station uses different frequencies to transmit tothe mobile stations. Again 125 channels are available for thetransmission of signals by the base station. N channels are also usedout of the available 125 channels for transmission by the base station.The frequency used to transmit to a mobile station is generallyseparated by a constant value from the frequency used by the mobilestation to transmit to the base station. In the following, reference isbeing made to traffic channels.

[0004] As a cell is served by a specific base station, all mobilestations within the cell transmit and receive signals at frequencieswhich have been allocated to the associated base station. A set of Ntransmit frequencies and a set of N receive frequencies are allocated tobase stations and mobile stations.

[0005] Each set of frequencies used in a particular cell need to bedifferent from the set of frequencies used in an adjacent or neighboringcell. This is to avoid co-channel interference. For example, if a firstcell used a frequency set comprising frequencies F₁, F₄, F₇ then alladjacent cells would use frequencies other than F₁, F₄, F₇.

[0006] With GSM there are 125 frequency channels available, as mentionedpreviously, which means that there will need to be the same frequencychannel allocated to more than one cell within the network. The cellswith identical frequency sets could be a large distance apart, forexample in the countryside where there tends to be fewer mobile stationssimultaneously requesting service. Conversely, the cells with identicalfrequency sets could be a very small distance apart, for example in amajor city centre where there tends to be a large number of mobilestations simultaneously requesting service. The neighboring cells whichare allocated different frequency channel sets are arranged together inclusters.

[0007] In areas where there is a large number of mobile users all tryingto communicate with the base station at the same time, for example inmajor conurbations, there is a need for a correspondingly large numberof frequencies to be allocated to each cell, in other words there is aneed for a large frequency channel set to be associated with eachheavily used cell. This is because many mobile users require servicefrom each base station at substantially the same time. Accordinglyfrequency reuse patterns are much tighter. That is cells using the samefrequency are relatively close together. Additionally, channels of asimilar frequency are likely to be present in adjacent cells leading toadjacent channel interference.

[0008] As the number of mobile users increases and the size of cellshave been made smaller, adjacent channel interference and co channelinterference has began to becomes more problematic. This is especiallythe case where the network is synchronized and cyclic frequency hoppingis in use. This is because one user will be effected by the sameinterferer at all times whereas if synchronization and cyclic frequencyhopping were not in use, the interferer would be randomly spread amongstall the users.

[0009] To mitigate co channel interference, the networks are currentlydesigned to have sparse reuse of frequency. This means that the distancebetween cells which are allocated the same frequency channels arerelatively long. This reduces interference within the network and alsomeans that adjacent channel interference is not generally problematic.However this does not allow the full capacity of networks to be realisedand as the number of subscribers expands and the size of cell reduces,this strategy is disadvantageous.

SUMMARY OF INVENTION

[0010] It is therefore the aim of embodiments of the present inventionto achieve a higher capacity.

[0011] According to a first aspect of the present invention, there isprovided a receiver comprising means for receiving a first plurality ofsignals at different frequencies at substantially the same time; andmeans for comparing a wanted signal at a first frequency with at leastone other signal at a frequency similar to that of said wanted signal toprovide a measure of interference provided by said at least one othersignal.

[0012] According to a second aspect of the present invention, there isprovided a network planner for controlling the frequencies used by atleast one network element, said network planner controller beingarranged to use information provided by at least one network elementrelating to the interference at said network element between at leastone desired signal and at least one unwanted signal at a frequencysimilar to that of said wanted signal, the network planner beingarranged to control the frequencies used by at least one network elementin accordance with said information.

[0013] According to a third aspect of the present invention, there isprovided a method comprising the steps of receiving a first plurality ofsignals at different frequencies at substantially the same time; andcomparing a wanted signal at a first frequency with at least one othersignal of a frequency similar to that of said wanted signal to provide ameasure of interference provided by said at least one other signal.

BRIEF DESCRIPTION OF DRAWINGS

[0014] For a better understanding of the present invention and as to howthe same may be carried into effect, reference will now be made by wayof example to the accompanying drawings in which:

[0015]FIG. 1 shows the layout of a typical cell network;

[0016]FIG. 2 shows how base stations and a base station controller areconnected;

[0017]FIG. 3 shows the layout of a typical cell network with a frequencyreuse of nine;

[0018]FIG. 4 shows a three site cluster comprising three base stations;

[0019]FIG. 5 shows a cell with three sectors and one base station;

[0020]FIG. 6 shows a 3/9 cell cluster comprising three base stations;

[0021]FIG. 7 shows a 1/1 reuse scheme;

[0022]FIG. 8 shows a block diagram of a base transceiver station;

[0023]FIG. 9 shows a base transceiver station embodying the presentinvention;

[0024]FIG. 10 shows a typical received signal at a base station;

[0025]FIG. 11 shows the output from the SAW filter of FIG. 9.

DETAILED DESCRIPTION OF EMBODIMENTS OF THE PRESENT INVENTION

[0026] Reference is now made to FIG. 1 which shows part of a cellulartelecommunications network 2 in which embodiments of the presentinvention can be implemented. The area covered by the network is dividedinto a plurality of cells, four of which are shown in FIG. 1. Each cellhas associated therewith a base transceiver station 4. The basetransceiver stations 4 are arranged to communicate with mobile terminals6 located in the cell associated with a given base station 4.

[0027] Embodiments of the present invention are primarily related to thefrequency reuse of traffic channels. However embodiments of theinvention are alternatively or additionally applicable to the reuse ofcontrol channels such as the BCCH channel or any other beacon or pilotchannel. In the case of GSM systems, some channels may be lost for BCCHduty.

[0028] The base stations are controlled by a base station controller, asshown in FIG. 2. Three base stations 12, 14, 16 are all allocated aparticular frequency set at which they operate. Each base station 12,14, 16 is also allocated an identity number (base station identity codeBSIC) with which it can be identified. The base station controller 10 isarranged such that it can communicate with the plurality of basestations 12, 14, 16. The base station controller 10 controls thefrequency sets used by the respective base station 12, 14, 16 so thatlocal area network planning can be achieved. In FIG. 2, three basestations are shown as being connected to the base station controller.More or less than three base stations may be connected to the basestation controller. In a network, more than one base station controlleris provided and these base station controllers communicate via anothernetwork element (not shown).

[0029] Referring now to FIG. 3 which shows a typical cell network 18having a frequency reuse of nine. The network 18 is composed of a numberof individual cells 20. Each cell has been assigned a set of frequencychannels for communications between the mobile stations and therespective base station. A frequency set therefore comprises thefrequencies with which the base station communicates with the mobilestation and the frequencies with which the mobile station communicateswith the base station. In FIG. 3 each frequency set f1, f2, f3, . . . ,f9 is composed of one or more of the receive frequency channels F1, F2,F3, . . . , F125 and the associated transmit frequencies. No frequencyappears in more than one set. The frequency sets f1, f2, f3, . . . , f9are arranged in respective clusters A and B such that there are a numberof adjacent cells, in this particular case nine cells, which contain thedifferent frequency sets. These clusters are repeated to cover thenetwork.

[0030] In FIG. 4 there is shown a three site cluster 22 with a frequencyreuse of three. The three frequency sets f1, f2 and f3 are allocated tothe respective cell sites 24. Again no frequency appears in more thanone set. The frequency channel sets f1, f2 and f3 may comprise all theavailable 125 frequency channels.

[0031]FIG. 5 shows a single base station site 28 which has been dividedinto three sectors. Each sector operates using a different frequencychannel set, more specifically sector one 32 operates using frequencychannel set f1, sector two 34 operates using frequency channel set f2and sector three 36 operates using frequency channel set f3. Thissectorisation is achieved by the base station 30 using directionalantennas positioned at, in this case, 120 degrees around the basestation. The base station 30 can in practice be regarded as three basestations, although some components will be common to all three sectors.It is possible that there may be more or less than three sectors in acell. There may only be three frequency sets f1, f2 and f3 with eachbase station site defining a cluster. Alternatively, the base stationsite may form part of a cluster with one or more other sites. This isshown in FIG. 6.

[0032]FIG. 6 shows a nine sector cluster 38 comprising three basestation sites. Each sector 40 of each base station site is arranged suchthat it operates using a different frequency set. Each base station site42, 44, 46 is arranged such that it controls three of the sectors 40.The first base station site 42 uses the frequency channel sets f1, f2,f3. A second base station site 44 uses the frequency channel sets f4,f5, f6. A third base station site 46 uses the frequency channel set f7,f8, f9. Again, no frequency appears in more than one set. One frequencyset is allocated to each site. This means that there is a frequencyreuse of nine in the cluster.

[0033] This kind of arrangement is useful in areas where there is a highdensity of subscribers. These sectorised cells are useful in heavilysubscribed areas because a single base station site serves, in thiscase, three sector cells. Each one of these sector cells has associatedwith it one of the nine frequency channel sets f1, f2, . . . , f9 and sotherefore there is greater potential to service an increased clustertraffic density. There is an increase in cluster traffic density becausethere is effectively nine small cells where originally there was onlythree cells, each one assigned to a different frequency channel set.

[0034] This kind of arrangement is an example of a 3/9 cell clusterbecause there are three base station sites controlling nine cells withina reuse cluster.

[0035]FIG. 7 shows an example of a 1/1 reuse scheme. A base station site50 is arranged to operate in three sectors 48. These sectors 52, 54, 56are arranged so that they each operate using different frequency channelsets f1, f2, f3. At substantially the same time, each sector 52, 54, 56is arranged to operate using a different one of the frequency channels.In other words a first sector 52 can operate using, say, frequencychannel set f1. A second sector 54 can operate using either f2 or f3 anda third sector 56 would operate using the frequency channel set which isnot operational in either sector one 52 or sector two 54. This meansthat all three channel sets f1, f2, f3 are operational in the three cellsectors 48 at substantially the same time although in each individualsector of the three cell cluster there operates different frequencychannel sets at the same time. The advantage of 1/1 schemes is thatthere is an increase in channel traffic density when compared with thepreviously described methods. This arrangement is particularly usefulwhere there is a very high density of subscribers requesting service atsubstantially the same time. This kind of arrangement requires that eachsector changes the frequency channel set with which it operates atsubstantially the same time so a synchronous network is required toensure that this criteria is met.

[0036] Two types of interference occur in a cellular network. The firsttype is co-channel interference. Co-channel interference is due to thefact that a cell or cell sector works on a particular frequency channelat any one time and a cell, which may be a non-adjacent cell, works atthe same frequency at the same time. This means that because the firstcell operates at the same frequency as the second cell, the first cellproduces co-channel interference in the second cell. Co channelinterference is a greater problem with smaller cells and/or reuseclusters.

[0037] The second type of interference is adjacent channel interference(ACI). As the number of mobile subscribers increase, the density ofusers requesting service in a particular cell is likely to increase.This means that there needs to be an increase in the number of channelsallocated to a particular cell. This in turn means that cells operatingusing adjacent-frequency channels also tend to be positioned closertogether. In other words, cells which use frequencies adjacent to eachother are placed a shorter distance apart when there is a higher demandfor service at substantially the same time. This means therefore thatthere tends to be a stronger signal received at a particular basestation which is at a frequency adjacent to the frequency channel atwhich the base station is operating. ACI also tends to limit the channelcapacity of a network although at a much reduced level compared to CCI.

[0038]FIG. 8 shows how a receiving part 60, a processing part 62 and acontrol part 64 interact in a base station as described in the preferredembodiment of the present invention. The receiving part 60 of the basestation is designed to receive a signal which may contain severalfrequencies at substantially the same time. This may comprise a form ofantenna or other suitable means of reception. The base station will thenprocess the received signals, using the processing section 62, in amanner which will allow not only the wanted signal to be extracted fromthe received signal and processed to allow communication to and frommobile stations but will also allow the adjacent channel interference tobe measured. The processing section 62 will pass this measuredinterference level, and other data to the control section 64 which willcontrol the base station so that it acts in response to the measuredinterference level. The processing section 62 will also relay themeasured data back to a base station controller which will receive suchdata from a plurality of base stations and will then control thefrequency allocation as on the basis of the information on theinterference levels received from the base stations.

[0039]FIG. 9 shows the arrangement of FIG. 8 in more detail. A basestation receiver 66 has a receiving antenna 68 arranged in such a waythat it can receive signals carried by M different frequency carriers atthe same time. Each of the M different carriers is at a radio frequency.M may be equal to N. Each carrier frequency channel has a bandwidth of200 kHz. Adjacent carrier frequencies F₁, F₂, . . . FM are spaced apartby at least 600 KHz. The M different carriers and the signals carriedthereby received by the antenna 68.

[0040]FIG. 10 shows a signal 86 which may be received at the antenna 68.The signal 86 received by the antenna 68 comprises a number of differentfrequencies all received at substantially the same time. A first wantedcarrier 88 is received substantially at the same as two adjacentcarriers A1 and A2. A second wanted carrier 92 is received atsubstantially the same time as two adjacent carriers A3 and A4. Thewanted carriers C1 and C2 received in the signal 86 may or may not havea larger signal strength than the received unwanted carriers A1 to A4.Signals A1, A2, A3 and A4 are intended for a different base station andare adjacent in frequency to the wanted carriers. By adjacent, it ismeant the next frequency channel next to the desired frequency channel.In FIG. 10, the unwanted carrier A3 has a larger amplitude than thesecond wanted carrier C2 which is adjacent to it. This means that thecarrier to interference ratio (CIR) is poor for the second wanted signalC2. The CIR is a measure of received signal quality, the higher the CIRthe better the quality of the received signal.

[0041] The output of the antenna 68 is connected to the input of a mixer72 which has attached to its other input a downconverter 70. Thedownconverter 70 oscillates at a substantially constant frequency Themixer 72 mixes the received input M frequencies from the receivingantenna 68 with the signal produced by the down converter 70 to produceM frequencies which are at lower frequencies than the received signals.The output from the mixer 72 may be at an intermediate frequency or at abase band frequency. The output from the mixer 72 is then amplified bythe amplifier 74 which then passes the output signal to a surfaceacoustic wave (SAW) 76 filter. This filter provides filtering of theinput signal so that only the signals of interest, more specifically thewanted signal C1 or C2 and the two adjacent carriers A1 and A2 or A3 andA4, are provided to the components downstream of the SAW filter 76.

[0042]FIG. 11 shows the output 94 of the SAW filter 76 when thefrequency of interest is the second wanted carrier C2. As can be seen,the only signals allowed to pass through the SAW filter 76 are thesecond wanted carrier C2 and the two carriers A3 and A4 adjacent to thesecond wanted carrier C2. This means that the other signals received bythe antenna 68 are filtered out by the SAW filter 76. These signalswhich are allowed to pass through the SAW filter 76 are passed on tocircuitry downstream of the SAW filter 76.

[0043] The output of the SAW filter 76 is input to an analogue todigital convertor (ADC) 78. The ADC 78 converts the analogue signalwhich is output from the SAW filter 76 into a digital representation ofthe SAW filter 76 output. To implement this, the ADC 78 needs to samplethe input signal at a very high frequency which is at least twice themaximum frequency present in the input signal. The output from the ADC78 is therefore a digital data stream which is a digital representationof the output of the SAW filter 76. The digital representation outputfrom the ADC 78 is input to a numerically controlled oscillator NCO 80which uses a numerical control signal to alter the frequency at whichthe NCO 80 oscillates. The NCO 80 is used to convert the digital signalfrom an intermediate frequency to a base band signal.

[0044] The output from the NCO 80 is a,t a base band frequency. Attachedto the output of the NCO 80 is a first band pass filter 82. The firstband pass filter 82 is arranged to filter out signals which fall outsidethe receive band in which the frequency channel of interest resides. Thefirst band pass filter 82 is about 180 kHz wide. The first band passfilter 82 is a narrow band filter which is used to extract the wantedsignal. The first band pass filter 82 is arranged so that, in this case,the second wanted signal C2 is allowed to pass through it substantiallyunattenuated and the remaining signals are filtered out. The adjacentchannel signals are removed. Additionally connected to the output of theNCO 80 is a second band pass filter 84. The second band pass filter 84is arranged to filter out signals which fall outside the receive band inwhich the frequency channel of interest resides. The second band passfilter is 600 kHz wide, which may be the width of the SAW filter 76. Thesecond band pass filter 84 has a wide enough band pass characteristic sothat the wanted channel and the two channels adjacent to the wantedchannel are allowed to pass through. The output from the second bandpass filter 84 is then passed to digital signal processing apparatus(not shown) to allow the strength of the carriers adjacent to the wantedsignal to be measured.

[0045] The digital signal processing apparatus calculates the powerpresent in each of the three carriers separately, in this case thesecond wanted signal C2 and the two carriers adjacent to it A3 and A4,which are presented to it. Once the signal strength of each of the threecarriers has been measured, the DSP apparatus compares the signalstrength of the second wanted carrier C2 with the first adjacent signalA3 and then compares the second wanted signal C2 with the secondadjacent signal A3. From these comparisons the adjacent channelinterference and the CIR for the wanted signal C2 is calculated.Although only the channels adjacent to the wanted signals are describedhereinbefore it should be appreciated that other unwanted signals orcombinations of the unwanted signals may also be measured and processedin a similar manner. For examples, non immediately adjacent channelscould be taken into account.

[0046] Once the CIR has been calculated, the CIR is passed to a basestation controller (BSC). As described in FIG. 2, the base stationcontroller controls a plurality of base stations and makes a decisionbased on the CIR as to how each individual base station should becontrolled and in particular the frequencies it should use. One way inwhich the BSC can act on the adjacent channel interference is if a highadjacent carrier signal strength is detected, in other words there is alow CTR. If this occurs then the BSC can allocate a different frequencychannel to the mobile station, hand control of the mobile station to amore suitable base station within the BSCs control or change thefrequencies used in an adjacent cell.

[0047] Another feature of the ACI measurement is that it indicates agood propagation path between the mobile station and the base station.This means that this data can be used to construct information about thepropagation environment around the base station. This is useful fordetermining the radio environment and can also be used in networkplanning.

[0048] A further feature of the adjacent channel interferencemeasurement is that it allows the BSC to decide what type of informationis allocated to each particular frequency channel. For example, in FIG.10 there is shown a typically received signal. As can be seen, the firstwanted carrier C1 has a higher CIR than the second wanted carrier C2.This means that the quality of the first wanted carrier is better thanthe quality of the second wanted carrier. The BSC could thereforeallocate a telephone voice channel to the second wanted carrier C2 and adata channel to the first wanted carrier C1. This is because the qualityof channel required to be a data channel is much greater than thatrequired for a telephone voice channel. This effectively allows the BSCto intelligently allocate different methods of communication to moresuitable frequency channels.

[0049] In a 1/1 synchronised scheme an adjacent or neighboring cell willbe operating at a particular frequency and at some point during thehopping sequence, the serving cell will then operate at the samefrequency as the adjacent or neighboring cell. This will consequentlylead to interference. In a 1/1 scheme as described hereinbefore,measurement of the adjacent channel interference can allow the basestation controller to calculate which mobiles are causing interferenceand which are being interfered with. The base station controller canconsequently instruct a base station site to alter the frequency hoppingsequence it uses to reduce this interference. Furthermore it can alsoforce handovers of mobile stations to other base station sites tomitigate the problem.

[0050] Whilst embodiments of the present invention have been describedin relation to a GSM system, embodiments of the present invention can beused with any other suitable standard using time division multipleaccess (TDMA), spread spectrum systems such as code division multipleaccess (CDMA), frequency division multiple access (FDMA), space divisionmultiple access (SDMA) and hybrids of any of these systems.

[0051] Embodiments of the present invention have been described in thecontext of a receiver for a base transceiver station. However,embodiments of the present invention can be used in any other suitablereceiver such as in a mobile station as well as other types of receiverwhich are not used in cellular networks but which are arranged toreceive a number of signals, at different frequencies, at the same time.Where the arrangement is included in mobile stations, the mobilestations may report back their readings to the base station controllervia the respective base station. Other applications may includemultipower control for transmitters.

1. A telecommunications network comprising: at least one networkelement; a network planner for controlling the frequencies used by atleast one of said network elements, said network planner controllerbeing arranged to use information provided by at least one of saidnetwork elements; wherein said at least one network element forproviding said information means comprises; means for receiving a firstplurality of signals at different frequencies at substantially the sametime, and means for comparing a wanted signal at a first frequency withat least one other signal at a frequency similar to that of said wantedsignal to provide a measure of interference provided by said at leastone other signal, and the network planner is arranged to control thefrequencies used by at least one network element in accordance with saidinformation.
 2. A network as claimed in claim 1 wherein comparing meanscomprises first filter means for removing the other signals except thefrequency of interest.
 3. A network as claimed in claim 2, wherein thecomparing means comprises second filter means for removing all othersignals except the frequency of interest and the at least one othersignal at a frequency similar to that of said wanted signal.
 4. Anetwork as claimed in claims 2 and 3, wherein said first and secondfilter means are in parallel.
 5. A network as claimed in any precedingclaim, wherein the comparing means comprises means for isolating thewanted frequency and at least one other signal at a frequency similar tothat of said wanted signal from at least some of the other receivedsignals.
 6. A network as claimed in claim 5, wherein said isolatingmeans comprises a filter.
 7. A network as claimed in claim 6, whereinsaid isolating means comprises a SAW filter.
 8. A network as claimed inany preceding claim, wherein the wanted frequency is adjacent to atleast one said similar frequency.
 9. A network as claimed in claim 8,where said two signals at similar frequencies are considered by saidcomparing means, one on either side of the frequency of said wantedsignal.
 10. A network as claimed in any preceding claim, wherein saidmeasure of interference is a carrier to signal ratio.
 11. A network asclaimed in any preceding claim, wherein said information relates to theinterference at said network element between at least one desired signaland at least one unwanted signal at a frequency similar to that of saidwanted signal.
 12. A method comprising the steps of: receiving a firstplurality of signals at different frequencies at substantially the sametime; comparing a wanted signal at a first frequency with at least oneother signal of a frequency similar to that of said wanted signal toprovide a measure of interference provided by said at least one othersignal; providing said measure of interference to a network planner;using said measure of interference to control the frequencies used inaccordance with said measure of interference.
 13. A network as claimedin any of the claims 1-11, wherein said network planner is a basestation controller.
 14. A network as claimed in any of the claims 1-11,wherein said network element comprises a base station.